A slip-protecting device is previously known, for example, through U.S. patent application Ser. No. 407,505, filed Oct. 18, 1973, now U.S. Pat. No. 4,035,698, issued July 12, 1977, which device is designed to be used in electrically operated vehicles, preferably railbound vehicles. This known slip-protecting device comprises slip-indicating members which sense the time derivative of the speed of the drive wheels and emit a slip-indicating signal when this derivative exceeds a predetermined value, which is chosen so that it is not reached during normal acceleration without slipping. This known slip protection is arranged so as to rapidly reduce the traction of the vehicle, upon incipient slipping, to zero or to a low value. Slipping is rapidly counteracted and when the drive wheels have regained adhesive contact with the rail traction is increased to its original value. During the time required for counteracting slipping, traction is reduced to a low value, in a typical case a few tenths of seconds or a few seconds. Each slipping therefore results in a loss of traction for a corresponding period of time. If the traction of the vehicle is set at a value which is too high in relation to the adhesive properties of the rails, slipping will occur relatively often, that is, the traction will vary between its maximum value and a low value. This can result in considerable reduction of the average value of tractive force.
It is therefore a prime object of the present invention to provide a motor-driven vehicle in which the above-mentioned drawback is avoided and by means of which a considerable increase of the available traction can be obtained, particularly in the case of poor adhesion conditions.